1. Heat the oven to 180 degrees C / 350 degrees F / Gas Mark 4.2. To make the lemon filling: In a medium saucepan, whisk together 200g caster sugar, flour, cornfour and salt. Stir in water, lemon juice and lemon rind. Cook over medium-high heat, stirring frequently, until mixture comes to the boil. Stir in butter. Place egg yolks in a small bowl and gradually whisk in 100ml of the hot sugar mixture. Then, whisk egg yolk mixture back into remaining sugar mixture. Bring to the boil and continue to cook while stirring constantly until thick. Remove from heat. Pour filling into pastry case.3. To make the meringue: In a large glass or metal bowl, whip egg whites until foamy. Add sugar gradually, and continue to whip until stiff peaks form. Spread meringue over pie, sealing the edges.4. Bake in preheated oven for 10 minutes, or until meringue is golden brown.